This invention relates to modulated carrier systems of the so called coded type in which digital symbols to be sent over a band limited channel are encoded as a sequence of discrete signal points selected from an available signal point alphabet, with dependencies being introduced between successive signal points in the sequence to increase immunity to noise and distortion.
In typical such coded systems, for example the systems described in Csajka et al., U.S. Pat. No. 3,877,768, and Ungerboeck, "Channel Coding with Multilevel/Phase Signals", IEEE Transactions on Information Theory, Vol. ITI-28, No. 1, January, 1982, information about the dependencies between successive signal elements is exploited at the receiver using a maximum likelihood sequence estimation decoding technique based on the Viterbi algorithm described in Forney, "The Viterbi Algorithm", Proceedings of the IEEE, 61(3):268 (March 1973), incorporated by reference. In such a technique, instead of decoding each received signal independently into the signal point most likely to have been sent (i.e., the signal point closest to the received signal in the sense of Euclidean distance) a sequence of received signals is decoded into the sequence of signal points most likely to have been sent (i.e., into the sequence of signal points closest to the sequence of received signals in the sense of the algebraic sum of Euclidean distances or vector Euclidean distance). In such coded systems, final decisions are delayed for a sufficient number of symbol intervals to assure to an acceptably high probability that the sequence of which signal points were sent will be correctly decided.
By contrast to coded systems, in uncoded systems each symbol is encoded into a signal point based only on the symbol to be encoded, with no dependencies between successive signal points. At the receiver, decoding proceeds one symbol at a time.
Receivers for uncoded systems typically include an equalizer to reduce the effects of intersymbol interference introduced by the channel, as described in Qureshi, "Adaptive Equalization", IEEE Communications Magazine, March, 1982, incorporated by reference. A so called linear equalizer for a quadrature amplitude modulation (QAM) system, for example, is typically a transversal filter which takes samples of a received signal, multiplies each sample by a complex coefficient, and adds the products to obtain an equalized received signal for use in deciding the most probable signal point to have been sent. For channels with severe amplitude distortion, such a linear transversal equalizer enhances noise and correlates the noise in successive intervals.
A decision feedback type equalizer (DFE) can be substituted for the linear equalizer to perform equalization with less noise enhancement. DFEs are described in the Qureshi article cited above, in C. A. Belfiore and J. H. Park, Jr., "Decision Feedback Equalization", Proceedings of the IEEE, August, 1979, and in D. D. Falconer, "Application of Passband Decision Feedback Equalization in Two Dimensional Data Communication Systems", IEEE Transactions on Communications, October, 1976, hereby incorporated by reference. Generally, a DFE multiplies previous decisions by feedback coefficients and sums the products to produce a value to be applied to the demodulated, equalized, undecoded received signal to correct for the anticipated channel intersymbol interference in the currently received signal.
Receivers for uncoded systems sometimes use modified DFEs (which may be called noise predictors) to predict and compensate for the error component in the received signal, as described in the Belfiore and Park article. The noise predictor output is a weighted sum of past error signals (each based on a comparison of a past received signal with the corresponding decision), where the weighting coefficients are selected to minimize the average power of the residual error signals after prediction by removing the correlation which exists between successive error signals before prediction. Unlike the conventional DFEs, in noise predictors the coefficients of the linear (or forward) equalizer are independent of the noise predictor (or feedback) coefficients. The forward equalizer coefficients can be updated to minimize the mean square error before prediction. FIG. 1 illustrates the use of a noise predictor having only a single predictor coefficient.
Other decision feedback techniques have been used to correct for other kinds of channel imposed distortion. For example, adaptive phase predictors correct for phase jitter using the history of actual phase errors reflected in the most recent decisions as an indication of phase distortion.
In using decision feedback techniques with uncoded systems, decisions of which signal points were sent are available without delay.
Use of the so called Viterbi algorithm as an optimum method of detecting a sequence of transmitted signals received over a noisy channel with a known pattern of intersymbol interference (ISI) is described in Forney, "Maximum Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference," IEEE Transactions on Information Theory, Vol. IT-18, No. 3, May 1972. This application of the Viterbi algorithm is similar to detecting a sequence of signals transmitted by coded modulation in that in both cases dependencies between successive signal elements are introduced by a finite state machine. In coded modulation systems, the finite state machine is the encoder in the transmitter, whereas in uncoded systems with ISI, the ISI model of the channel (including transmit and receive filters) represents the finite state machine as explained in the Forney article.
The complexity of the Viterbi algorithm when applied to uncoded systems with ISI increases exponentially with the number of ISI terms in the channel model. Therefore, methods of equalizing the channel to a desired impulse response with a relatively few number of ISI terms have been proposed by Qureshi and Newhall, "An Adaptive Receiver for Data Transmission over Time Dispersive Channels," IEEE Transactions on Information Theory, Vol. IT-19, No. 4, July 1973, and by Falconer and Magee, "Adaptive Channel Memory Truncation for Maximum Likelihood Sequence Estimation," Bell System Technical Journal, Vol. 52, November 1973. Both of these articles suggested using a linear transversal equalizer to equalize the channel to a few desired ISI terms prior to detection by the Viterbi algorithm.
When the amplitude spectrum of the original channel response and the desired truncated response are not similar in shape, equalization to the desired truncated response can be performed with less noise enhancement by using a DFE. Such a system suffers, however, from the same difficulty incurred in a coded system because the final decisions from the Viterbi algorithm are available for feedback only after a delay of more than a dozen symbol intervals.
Two schemes for using a DFE in conjunction with the Viterbi algorithm have been described in Qureshi, "An Adaptive Decision Feedback Receiver Using Maximum Likelihood Sequence Estimation," IEEE International Conference on Communications Record, 1973, and in Lee and Hill, "A Maximum Likelihood Sequence Estimator with Decision Feedback Equalization," IEEE Transactions on Communications, Vol. COM-25, No. 9, September 1977.
In the Qureshi scheme, the feedback information is tapped from the most likely path history of the VA (Viterbi algorithm) after some delay. The greater this delay is, the smaller the additional performance advantage of the DFE compared to a linear equalizer. However, the smaller the delay, the higher the probability of error in the tentative decisions being fed back which in turn leads to error propagation. Lee and Hill use an ordinary DFE with a memoryless threshold detector to obtain preliminary decisions. After a delay corresponding to the number of ISI terms in the desired truncated response, these decisions are used in the DFE which truncates the channel response to the desired length for the VA. Both the Qureshi and the Lee and Hill schemes exhibit error propagation in the DFE due to feedback of preliminary decisions with a probability of error higher than the final delayed decisions. In each case, a single equalized signal is used as input to the VA.